The lack of low-cost access to orbit is the greatest detriment to the near-term exploitation of space. The ability to deliver payloads to orbit for substantially less than $5,000 per pound is yet to be realized. This results from the expendable nature of the current stable of space launch vehicles. The space shuttle being a combined space launch vehicle and manned orbital laboratory and workshop, while reusable, represents a more costly approach to payload delivery. Analysis of actual space launch vehicle delivery costs to date indicate that both the vehicle costs and the mode of space operations must be modified.
Numerous conceptual design studies have been performed since the mid-1960's, attempting to find a solution but none (other than the partially reusable space shuttle) have demonstrated sufficient cost leverage and utility to move forward into development. All the fully reusable concepts produced very large vehicles or lacked adequate credibility. The current U.S. main effort is the National Aerospace Plane (NASP) which would employ slush liquid hydrogen fuel and atmospheric oxygen as the propellant. Results to date indicate that advanced technology is required in all technical disciplines and vehicle systems especially in the combined-cycle engine. Because of the low density of the hydrogen fuel and drag losses from the required flight in the atmosphere, the resulting airframe is quite large.
The challenge facing the reusable single-stage chemical rocket space-launch vehicle designer is illustrated in a relationship derived from the rocket equation expressed as: EQU .DELTA.V=g.times.Isp.times.log (W.sub.initial /W.sub.final)
where .DELTA.V is the velocity delivered by the rocket engine to achieve orbit, g is the acceleration of gravity, and Isp is the specific impulse of the rocket engine.
Assuming .DELTA.V (including all losses)=32,500 feet/sec., vacuum Isp=460 sec. for a liquid oxygen-liquid hydrogen engine, and a fixed payload weight=10,000 lbs., the following relationship for the launch weight results: EQU W.sub.launch =90,000.times.[(W.sub.empty /W.sub.payload).sub.min +.DELTA.W.sub.empty /W.sub.payload +1]
where W.sub.empty is the vehicle burnout weight at orbit insertion for no payload, and .DELTA.W.sub.empty represents the weight increment attributed to a non-optimal concept. The weight .DELTA.W.sub.empty includes both the weight of the non-optimal subsystem carried into and returned from orbit and the vehicle empty weight growth resulting from the added propellant and engine thrust required. An example would be the wing required for airfield takeoff but not required for reentry and landing. The feasibility and cost of the fully reusable space launch vehicle is directly dependent on the vehicle empty to payload weight ratio. For a value of 20, a 2 million pound launch weight is indicated and an empty weight growth of 5 percent for launch weight fixed drops the payload weight to zero. For system feasibility, a vehicle empty weight to payload weight ratio of 10 or less is probably required. One suggested approach would follow the vertical launch technique and rely on advanced materials technology to reduce the empty to payload weight ratio to about 10. However, this approach is paced by the availability of advanced technology development resources. A more challenging problem for the designer is to search for configuration options and mode of flight options that can make for a near-term, low-cost solution. (The classic example of this type of approach was the discovery and implementation of lunar orbit rendezvous which made possible manned landing on the moon in the 1960's). In all probability, more than one breakthrough together with favorable synergism between concepts will be necessary. One recent example of a new approach put into practice was the successful flight of the small Pegasus space launch vehicle which employed a B52 aircraft as the first stage which dropped a three-stage vehicle having a winged initial stage to provide an aerodynamic turn to transition from horizontal to climbing flight. Many studies exploring aircraft launch have been conducted for large payload space launch vehicles mainly to exploit the flexibility of an airfield launch site. The primary stumbling block has been the lack of a suitable heavy payload launch aircraft.